Gain control circuits



Oct. 27, 1936. H. FRIEDRICH 2,058,972

GAIN CONTROL CIRCUITS Filed May 14, 1954 704 a) 70 PM. 5- 7 70/55 04% M55 INVENTOR jig FgED/P/CH AT ORN EY I UNITED s'rr I. ATET GAIN CONTROL CIRCUITS Hans Friedrich, Berlin, Germany, assignor to Telei'unken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany Application May 14, 1934, Serial No. 725,484 In Germany May 2, 1933 6 Claims. (Cl. 250-20) The present invention is concerned with a circontrol grid. In this method, two disadvantages cuit scheme which is adapted to insure automatic become evident. In the first place, there occurs a gain or volume control, comprising the use of marked fluctuation of the means anode current electron tubes furnished with five or more elec- Ja, as can be readily seen from Fig. l. Jmi is trodes. the mean anode current, in the presence of low 5' A necessity for amplification control arises in grid biasing voltages and high gain, while Jmz connection with various kinds of work. Known is a working point corresponding to a high grid from the prior art, for instance, is the control or biasing potential and small slope. In the second regulation of the gain in receiver apparatus which place, a relatively high regulating voltage is re- 10 are to produce a stable and constant acoustic quired in order to be able to cover the entire range 10 volume in spite of variations in the input voltage. of the slope.

For other reasons regulation of the volume is re- Now, the purpose of the present invention is quired, e. g., in making disk records for phonoto obviate the said two drawbacks by the aid of a graphs, in the photographic recording of sound novel type of circuit arrangement for a tube conand in the modulation control of transmitters. taining five or more electrodes. Referring to 15 The volume of sound of an orchestral program Fig. 2, V represents an amplifier tube comprisvaries roughly inside the limits between 1 and ing a directly heated or an indirectly heated cath- 1000. In the recording of phonograph records Ode K, an anode or plate A, a control grid Gl, a there exists a lowermost acoustic volume limit screen grid G2, and an additional g id G3 (cath- 25 or level which is imposed by the needle noise and de 0r upp s g d)- Plotting the characterbelow which one should not go. The uppermost istic curve of such a tube in which the anode or limit is fixed by the inter-groove or track displate current Ja is plotted against the grid volttance. However, in this manner it is possible only age E471, with the voltage Egg being negative and to arrive at an acoustic-volume ratio of about constant foragiven characteristic curve, there re- 1:50. suits a family of characteristics of different slopes 25 Hence, it is necessary to diminish the natural Or mutual CO d ta C W c i ec psound-volume ratio between the lowest and the proximately at the point P of the abscissa axis, loudest passages or notes (socalled volumeas shown in Fig. 3. The slope of the various ratio) from 121000 to a value around 1:50. If the characteristic curves is here a function of the assumption be made that the median amplitude Voltage of the third grid. This indicates a chance shall not be affected by the regulation or control of regulating the gain by variations of the voltage action, it follows that subdued passages must be Eg's as a function of the amplitude of the input amplified more, comparatively speaking, than alternating voltage.

loud passages. The situation is similar, for in- Inasmuch as the characteristics are essentially 3'5 stance, in the modulation of a broadcasting starectilinear over a major portion thereof, there re- '35 tion; the percentage modulation in the case of sults this advantage that, also for large amplilow passages, with due regard for the interfering tudes, no d amplification is p a e, or side (background) noises in the receiver apwhile in the case of the exponential curves shown paratus and atmospherics should not be too low, in Fig. 1 this is feasible only to a substantially 40 while, on the other hand, all chances of overlower degree. It is also found that the regulat- 0 modulation should be carefully avoided on acing voltage which is required for gain control becount of the distortions associated therewith. tween definite limits, is considerably lower in the Figures 1, 3, and 6 are curves illustrating the present novel mode of slope variation than in the invention. case of exponential tubes (e. g., 5 volts compared Figures 2, 4, and 5 are fragmentary circuit diawith 30 v.). 45 grams embodying the invention. In order to explain more clearly the effect of Regulation of the gain has heretofore been ina change in the mean plate current as a result of sured principally in amplifier stages containing gain control, reference is here made to Fig. 4, exponential-type tubes especially suited for this Where V is the regulated amplifier tube, as expurpose; in other words, tubes whose characterplained in more detail by reference to Fig. 2, istic curve, as shown in Fig. 1, have an approxiwhereas V is the end or power stage of the mately negative-logarithmic form and which examplifier. For all of the tubes there is provided tends far into the negative grid biasing voltage a joint anode source Ea. Although the resistance region. Regulation is insured, as Well known in of this source of potential is comparatively low,

the art, by reduction of the biasing voltage of the and although it is further reducible by the parallel 55 connection of a blocking condenser C, there nevertheless happens a reaction upon the input stages by way of the voltage source, because of the size of the alternating voltage amplitudes arising in the end or power stage, unless special filter sections L and C are connected in the plate lead of the input stages. This filter chain has a timeconstant or resonance frequency governed by its dimensions, and this is an impediment to a quick change in the mean plate current of the regulator stage V. By the adoption of a number of steps it has been ascertained that the transition from one extreme of the amplification or gain to the other must be accomplishable inside an extremely brief space of time, lest appreciable distortion become noticeable.

This drawback is obviated according to this invention by that the screen grid G2 is connected with the plate potential. The grid G3 insures a control action, without itself consuming any current, upon the distribution of the current emitted from the cathode between the screen grid G2 and the anode A. Hence, if for instance, the mean plate current is diminished as a result of an increase in the negative biasing voltage of the regulator grid G3 (curve Ega in Fig. 3), then the screen grid current will grow a similar amount. Consequently, the aggregate current Ja+J2 (Fig. 2) derived from the plate-current source remains approximately constant, and the time constant of the filter LC plays no part at all.

The operation and effect of the regulating stage before described is still further improvable by the use of a six-electrode tube (hexode) being used in a circuit scheme as shown in Fig. 5. In addition to the cathode K and the anode A, this tube contains further four grid-type electrodes. GI designates again the control grid upon which is impressed the input voltage Eg to be amplified. The grids G2 and G3 both act as screen grids, whereas the grid G4 is employed to act as a regulator electrode in a way previously described. Upon the second screen-grid G3 is again impressed the plate direct voltage, whereas the first screen grid G2 is advantageously connected with a lower positive voltage. Owing to the fact that ahead of the grid G3 there is mounted a further screen grid, a still better stabilization of the aggregate current which is taken from the platepotential source is secured.

This is illustrated by Fig. 6 where the current J3 of the second screen grid G3 and the plate current J a is plotted in dependence upon the regulator voltage Egi. There is moreover a chance to insert in the circuit of the first screen grid G2 a useful resistance W which may be rendered serviceable for various purposes. For instance, at this particular place a supervising (monitor)- telephone may be inserted, or else the resistance W may serve for the connection of a branch amplifier which serves either for controlling or supervising purposes, or which may be used in combination with a rectifier for furnishing the regulator voltage Egs. It goes without saying that the circuit scheme here disclosed may analogously be applied to push-pull stages whereby non-linear distortions are avoidable.

While I have indicated and described several arrangements for carrying my invention into efiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular constructions shown and described, but

that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What is claimed is:

1. In combination, a source of electric waves, a wave transmission tube provided with a cathode, anode and at least three grids interposed between them, said source being connected to said cathode and one of the grids, means for maintaining said one grid at substantially constant negative direct current potential, a source of direct current voltage, means for connecting the said anode and a second one of the grids to a common positive voltage point on said second source, means for varying the potential of the third of the grids with a direct current voltage in response to changes in amplitude of said waves, said third grid being disposed adjacent the said anode, and said second grid being disposed between the first and third grids whereby variation of said third grid potential has substantially no effect on the total current flowing to said plate and second grid from said direct current source.

2. In combination, a source of electric waves, a wave transmission tube provided with a cathode, anode and at least three grids interposed between them, said source being connected to said cathode and one of the grids, a source of direct current voltage, means for connecting the said anode and a second one of the grids to a common positive voltage point on said second source, means for varying the potential of the third of the grids with a direct current voltage in response to changes in amplitude of said waves, said third grid being disposed adjacent the said anode, and said second grid being disposed between the first and third grids, and an auxiliary electrode disposed between the first and second grids, the electrode being maintained at a positive voltage which is lower than that of the said anode.

3. In combination, a source of electric waves, a wave transmission tube provided with a cathode, anode and at least three grids interposed between them, said source being connected to said cathode and one of the grids, a source of direct current voltage, means for connecting the said anode and a second one of the grids to a common positive voltage point on said direct current source, means for varying the potential of the third of the grids with a direct current voltage in response to changes in amplitude of said waves, said third grid being disposed adjacent the said anode, and said second grid being disposed between the first and third grids, said varying means comprising an auxiliary electrode disposed adjacent the said first grid, and a wave rectifier circuit operatively associated with said auxiliary electrode and said third grid.

4. In an electric wave amplifying system of the type including a source of waves and a tube provided with a cathode, an anode and four successive grids disposed between the anode and cathode, means for maintaining the anode and two of the grids at positive potentials, said source being connected between the cathode and a third grid, and a wave rectifier circuit having its input coupled to one of said positive grids to derive wave energy therefrom for rectification, and means for connecting the fourth grid to a direct current voltage point in the rectifier circuit whose potential magnitude is a function of the wave amplitude.

5. In a system as defined in claim 4, the fourth grid being disposed adjacent the anode and between the latter and one of the positive grids.

6. In combination in an audio frequency amplifier, a source of waves to be amplified, a wave amplifier tube provided with a cathode, an anode,

and at least three control electrodes interposed between them, said wave source being connected to the cathode and one of the control electrodes, a source of direct current voltage, means for connecting the said anode and a second one of said control electrodes to a common positive potential point on said second source whereby they are maintained at the same positive direct current voltage with respect to the cathode, said one control electrode being disposed between the cathode and said second control electrode, the

third control electrode being disposed between the second control electrode and said anode, means, responsive tovariations in amplitude of Waves from said source, for varying the direct current potential of said third control electrode over a range of potentials which are negative with respect to said cathode, and means for maintaining said first control electrode at a substantially constant negative direct current potential with respect to the cathode.

HANS FRIEDRICH. 

